The present invention generally relates to medical diagnostic imaging and in particular to a method and apparatus for determining dynamic range of a digital medical image to be displayed.
X-ray imaging had long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial and abdominal images that often include the information necessary for a doctor to make an accurate diagnosis. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against an x-ray sensor as an x-ray technologist positions the x-ray sensor and an x-ray source at an appropriate height. The x-ray sensor then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system scans out the detected x-ray energy and prepares a corresponding diagnostic image on a display. Optionally, the x-ray sensor may be a solid state digital image detector. If the x-ray sensor is a conventional screen/film configuration, the screen converts the x-rays to light, to which the film is exposed.
In conventional radiographic imaging systems, the x-ray technique is chosen by the operator. The operator or the automatic exposure control system selects or determines a desired exposure for the selected screen/film configuration in order to obtain a desired optical density of the exposed film. The optical density represents the “lightness” or “darkness” of the screen, detector or film once exposed to x-rays. By controlling the manner (e.g., time, orientation, etc.) of exposure by the detector, screen or film to x-rays, the film lightness or darkness may be varied. It is preferable to achieve a consistent optical density from one exposure to the next in order to facilitate diagnosis and examination by physicians when analyzing radiographic images. Different exposures arise from one patient to the next, from one film type to the next, from one medical imaging system to the next, from one orientation to the next and the like.
In the past, it has been quite difficult to maintain a uniform optical density from one exposure to the next (e.g., patient to patient, film to film, system to system, patient angle to patient angle) due to inherent differences. For instance, each patient has a slightly different size and anatomy which causes the internal organs of the patient to be located at different positions relative to the detector or screen/film. For example, when attempting to obtain an x-ray of a chest image, every patient's lungs and rib cage are of a different size. The position of the lungs is also somewhat unknown which creates a large variance in the resulting exposure. Further, patient position is not precisely controlled and hence each patient is located in a slightly different position or orientation with respect to the detector or screen/film configuration. Variation in patient position and orientation further create variance in the resulting exposure. Optical density may further be varied due to the particular pathology followed by the x-rays through the patient, due to foreign objects within a patient (e.g., pacemakers and the like) as well as due to differences in patient thickness and resulting scatter pattern properties.
An automatic exposure control has been proposed for use with radiographic systems in an attempt to control the optical density of the exposed film. Automatic exposure control systems typically use an x-ray sensitive ion chamber located proximate the detector, screen/film configuration and arranged to be proximate a particular anatomy of a patients during examination. For instance, an ion chamber may be located within a region of the detector or screen/film configuration calculated to be proximate the patient's lung during a particular form of examination. Alternatively, or in addition, an ion chamber may be located proximate the patient's mediastinum. The automatic exposure control measures the x-rays detected by the ion chamber and terminates the exposure when a preset dose is measured.
However, automatic exposure control systems have experienced difficulties. In particular, the position of an individual patient's lung is unknown at the time that the ion chamber is placed proximate the detector, screen or film. Hence, different patients continue to create a large variance in the resulting exposure to the ion chamber. For instance, the ion chambers may not actually be located proximate certain patient's lungs or mediastinum. When an ion chamber is located proximate an anatomy other than the lung or mediastinum, the automatic exposure control terminates exposure based on inaccurate measurements. A certain percentage of chest films result in creation of either too dark or too light of an image. When the image is too dark or too light, it may be necessary to repeat the x-ray examination to retake the medical image. It is quite time consuming to retake medical images. Film development may require a relatively long period of time, such as five to fifteen minutes, during which the patient may leave the image acquisition area.
Further, a resulting presentation of a medical image is determined by the selection of the type of detector, film/screen configuration in combination with the desired x-ray technique. Different types of detectors and screens/film configurations experience different amounts of image noise. In the past, noise has been partially corrected by varying the input exposure time. However, to maintain a constant optical density from one exposure to the next, when detector, film or screen types are changed, the exposure time must be changed in order to account for the fixed dynamic range of the new detector, screen/film configuration. It is quite cumbersome to change detectors, screens or films, and thus rarely done.
More recently, digital detectors have been proposed for use with radiographic imaging. Digital detectors afford a significantly greater dynamic range than convention screen/film configurations, typically as much as two to three times greater. Heretofore, the automatic exposure control and operator must still be relied upon to limit the exposure of the digital detector to account for the detectors greater dynamic range.
A need remains for an improved dynamic range detection and control method and apparatus for use with digital medical imaging, such as in radiographic imaging.